1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine using an electrophotographic system or an electrostatic recording system, and particularly to what we call a cleanerless system that collects residual toner on an image carrier into a developing device for reuse.
2. Description of the Related Art
In recent years, image forming apparatuses such as copying machines have been progressively downsized. There has been, however, a limit to only downsizing units for carrying out charging, exposing, developing, transferring, fixing, and cleaning processes. In addition, in an image forming apparatus using a system to transfer a toner image formed on an image carrier to a recording material, a cleaner collects residual toner on the image carrier and disposes of it as waste. It is not environmentally preferable, however.
Therefore, there has been suggested what we call a cleanerless system that collects residual toner on an image carrier into a developing unit for reuse. This system collects the residual toner on the image carrier into the developing unit in a process of visualizing an electrostatic image on the image carrier by means of the developing unit to reuse the collected toner for forming another image. In the process of visualizing the electrostatic image on the image carrier by means of the developing unit, the developing unit is provided with a developing bias including an alternation of a voltage generating an electric field in a direction where the toner moves from the developing unit to the image carrier (hereinafter, referred to as development accelerating voltage) and a voltage generating an electric field in a direction where the toner returns from the image carrier to the developing unit (hereinafter, referred to as a collecting voltage). The residual toner on the image carrier is collected to the developing unit by means of a potential difference (hereinafter, referred to as a V-back potential difference) between the collecting voltage of the developing bias applied to a developing sleeve of the developing unit, which is a developer carrier, and a potential of an image carrier surface charged by a charger.
As a method of charging the image carrier, a contact-type charging unit has been gradually put to practical use since advantages of low ozone and low power are achieved: in the contact-type charging unit, a charging member (contact charging member) having a voltage (charging bias) is contacted against a photosensitive member as to charge a surface of the photosensitive member at predetermined potential and polarity. Particularly, a roller charging type charging unit using a charging roller (conductive roller) as a charging member is preferable in stability of charging and it is in popular use.
When forming an image having a high image ratio (a ratio of the number of dots of an image formed on the image carrier to the highest number of dots that can be formed in an image area on a recording material) thereof, however, the image formation requires a large amount of toner and it inevitably leads to a large amount of transfer residual toner every time an image is formed. In this condition, a part of the transfer residual toner is not collected to the developing device, thus causing a problem that the toner adheres to a non-image area.